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arXiv · 2511.13562

Bimodular Gravity: Vacuum Evolution with a Frame-Dependent Phantom Crossing

James Hallam·João Magueijo
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Abstract

Unimodular gravity recasts the cosmological constant as an integration constant, fixed by a constraint on the volume element rather than chosen in the action. We ask what becomes of this constant when matter couples not to the gravitational metric, but to a second metric disformally related to it through a scalar field. Imposing a volume constraint on each metric, we find a theory in which the scalar does not propagate, yet still drives a non-trivial expansion. Written as a single-metric theory, its kinetic term is fixed to a prescribed function of spacetime, and the cosmological constant is replaced by a vacuum contribution that need not be constant. Moreover, we find that this theory admits a phantom crossing through a purely frame-dependent mechanism. This construction, however, rests on a feature invisible with a single metric, and unimodular formalisms that are classically equivalent in that case cease to agree once there are two disformally related metrics.

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James Hallam, João Magueijo. 2026-06-23. Bimodular Gravity: Vacuum Evolution with a Frame-Dependent Phantom Crossing. https://arxiv.org/abs/2511.13562

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